In the current magnetic storage apparatus, a write/read composite magnetoresistive head is used in which writing is performed by using an inductive recording head and reading is performed by using a magnetoresistive head. A magnetoresistive film where the electrical resistance changes depending on the external magnetic field is adopted in a magnetoresistive head, and a spin-valve is employed in which a so-called giant magnetoresistive effect is used, i.e. a magnetoresistive effect created by stacking a ferromagnetic film and a non-magnetic film. The spin-valve has a structure consisting of antiferromagnetic film/ferromagnetic film (pinned layer)/non-magnetic intermediate film/ferromagnetic film (free layer). Since current usually flows in the plane of the spin-valve in the magnetoresistive head, it is called a CIP-GMR.
Moreover, in recent years, in order to increase the sensitivity of the magnetoresistive film, a CPP-GMR where current flows perpendicular to the film surface of the spin-valve and a tunneling magnetoresistive film (TMR) having a structure of antiferromagnetic film/ferromagnetic film (pinned layer)/barrier/ferromagnetic film (free layer), in which current flows perpendicular to the film surface, have undergone research.
As mentioned above, in either CIP-GMR, CPP-GMR, or TMR, an antiferromagnetic film and a ferromagnetic film are stacked and exchange coupling between them is utilized. Therefore, the characteristics of the antiferromagnetic film are very important and various methods have been disclosed for improvement of the characteristics as shown in Japanese Patent Application No. 171010/2002 (“patent document 1”), Japanese Patent Application No. 25822/2002 (“patent document 2”), and Japanese Patent Application No. 94141/2002 (“patent document 3”).
Patent documents 1-3 disclose a MnPt film where ordered structure is necessary, and, in the case of the MnPt film where ordering is necessary, it is described that the lattice distortion during transformation of the crystal structure has to be relieved in order to improve the exchange coupling. As a method thereto, patent documents 1 and 2 pay attention to the twin boundary formed during transformation of the crystal structure, in which crystal growth is symmetric through the grain boundary. These documents describe that the exchange coupling becomes greater by controlling the bicrystal grain boundary so that they are not parallel to the interface between the MnPt and the ferromagnetic layer. Moreover, patent document 3 discloses a method in which the lattice mismatch between the MnPt and the ferromagnetic layer are caused by displacement of the crystal plane and in which the lattice distortion during transformation of the crystal structure is not allowed to generate.
The control of these twin boundaries and the crystal plane of MnPt can only be controlled by film preparation conditions. The direction and angle of the twin boundary and the crystalline orientation are formed by the transformation of the crystal structure during heat treatment, making it difficult to control these preparation conditions.
As mentioned above, in either CIP-GMR, CPP-GMR, or TMR magnetoresistive head, an antiferromagnetic film and a ferromagnetic film are stacked and the exchange coupling between them is utilized. When increasing the areal recording density of the magnetic storage apparatus, the head is downsized, thereby, the characteristics of the antiferromagnetic layer are deteriorated and a decrease in the exchange coupling and dispersion thereof are created by the damage during processing (for instance, Applied Physics Letters, Vol. 83, p. 401 (2003)) or by decreasing the volume of the antiferromagnetic film. This may lead to a decrease in the yield of the head and to deterioration in reliability.